Calling Electrical Boffins!
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Calling Electrical Boffins!
Guys and Gals (ATPL students or current engineer/pilots). .. .I need some help please. I am just doing some PPL revision in preparation for practical flying, and have a question. I am not sure whether I am going too deep here for PPL studies, but I have been reading and reading and reading and getting interested. I must point out that electrics is not my strong point!. .. .With reference to AC generators I would like to know if I have understood the following about external excitation.. .. .Am I correct in assuming that a Generator winding and core that is externally excited via the battery source to create the electro magnet is constantly fed with electrical current from the battery to excite it, or does it become self exciting once the engine has been started? My train of thought, is that, if the battery failed the external excitation source would fail, therefore the alternator would not provide the output voltage to the bus bar and electrical power would be lost all together, is this correct?. .. .I hope the above makes sense! Any responses would be greatly received.. .. .Thank you.. .. .CAT 3C AUTOLAND.. .. .PS. I also posted this in the questions forum, but thought it may be worth posting here aswell <img border="0" title="" alt="[Smile]" src="smile.gif" />
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Hi,. .A simple answer to your question is to look at the system on a large aircraft..... .. .The jet engine normally has a generator attached to it's own ancillary gearbox, as the engine is started, it rotates the gearbox and the gene' rotates also. The gene' gets "excited" and produces current, fed to the Voltage Regulator etc etc. If the battery fails the engines still rotate and the gene's are still producing power...all very well until the battery dies completely and the DC control circuits drop out and thats when you normally lose the AC and control!. .. .Hope this quick answer helps but you might be going a little too deep for PPL!. .Regards Rodders <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />
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Not going against what rodders says about big jets. . .. .But you can also get self exciting alts which have an exciter on the shaft.. .This takes the form of a DC generator using perm magnets, so as long as the shaft is rotating there is a DC supply.. .. .But as rodders says its way to deep for PPL and you will get a 2" thick manual on this stuff if you do your ATPL exams. Both oxford and bristol ground school notes are pretty good on it.. .. .If you have a look at this web page. .. .<a href="http://www.perr.com/gen.html" target="_blank">http://www.perr.com/gen.html</a>. .. .under generator basics there is a good description how the thing gets its self going.
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From how I understand Trevor Thom (see vol4, p225):. .. .1. if your plane has a generator, it is fine producing electricity without any excitation current. .. .2. an alternator (which most planes have these days), it requires excitation, and "even if you hand-swing the propeller, the alternator won't come on line unless the battery has at least some residual voltage"
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The alternator shaft (in your car or simple aircraft) has a magnet on it which rotates. In so doing it causes a continuously changing magnetic field to pass through the coils surrounding the shaft. This will cause a voltage to appear in these coils which itself varies (alternates). In most practical alternators, this magnetic field is produced by a rotating coil of wire fixed on the shaft (i.e. an electromagnet). The magnetic field it provides must be constant hence the current must pass through this coil in one direction only (D.C.) Direct current is produced by batteries and used by the systems on cars and light aircraft. It is also needed to charge these batteries so the alternating current produced by the varying voltage generated in the coils surrounding this rotating magnet is converted into direct current by passing it through rectifiers. To get the alternator going, dierct current must be provided by the battery, however once it is generating, the D.C. it produces can be used to generate the required magnetic field (i.e. it self-excites.) Anybody who has had to hand swing a light A/C due to a flat battery will know that if there is a little life in the battery, the alternator will start working and the ammeter shows a charging current flowing. If the battery is totally "dead" however the alternator does not work, the ammeter needle stubbornly remains on zero and an immediate shut town is the order of the day so that the battery can be replaced or recharged. If a battery fails in flight however, the electrical system will often continue to work as the alternator(s) can self excite but on shutdown the engine cannot be restarted.. .. .Oh Lor' I wish this made it sound as simple as it is!. . . . <small>[ 13 March 2002, 21:18: Message edited by: P.Pilcher ]</small>
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Rodders, MJ, High Peak thanks very much I appreciate your input it all inter relates with Mr P.. .. .Mr Pilcher your vital source of information has made the penny drop <img border="0" title="" alt="[Smile]" src="smile.gif" /> . The DC produced by the Alternator through the Transformer Rectifier untit will self exite the field winding, hence not needing the DC from the battery (which is only used to get it going). Thanks very much, that has been giving my small brain pain for the afternoon, like you say so simple.....mmmm what does that say about me!!? <img border="0" title="" alt="[Eek!]" src="eek.gif" /> . .. .Cheers, much appreciated <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />. . . . <small>[ 14 March 2002, 08:39: Message edited by: CAT3C AUTOLAND ]</small>
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Most people forget that on a light airoplane there are two methods of power generation used. one is the alternator which only supplies power for the ancillary equipment and the other is the Magneto which provides the spark to the engine.. .. .The magneto is a good bit different to an alternator and the only link I know of that would give an explaination of how they work is <a href="http://old-engine.com/maghma.htm" target="_blank">here</a> .. .. .The magneto can work without any power being present in the battery which is why you can hand start engines on planes with flat batteries and why if the alternator fails you can turn everything but the magnetos off and still fly to your destination preserving your battery for when you need to make radio calls and such.
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Oh Bloody hell magnetos!! <img border="0" title="" alt="[Eek!]" src="eek.gif" /> . .. .Remember doing these, always used to confuse myself with the open circuit and closed circuit, switch open, circuit active, switch closed circuit dead <img border="0" title="" alt="[Smile]" src="smile.gif" /> .
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After discussing this at length with some other mech engineers we reckon that there should be enough residual field left in the crank to start the alternator up.. .. .After managing to restrain ourselves from going out to the apron and bastardising a cessna 150. We have decided that the only way to satisfy ourselves is to get a old alternator rigged up on a bench. So once we managed to get a decent 110v hilty power drill i will let you know what happens.. .. .MJ
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"If the battery fails the engines still rotate and the gene's are still producing power...all very well until the battery dies completely and the DC control circuits drop out and thats when you normally lose the AC and control!". .. .Not sure which big jets you're talking about" Rodders", but on the 747-400, the primary source of power for the DC control circuits (in the GCU's), is the permanent magnet generator on the Gen (The Main Hot Battery Bus only provides backup power to the GCU's: If the generators are on line, there should be a constant supply of power to this bus anyway).. .. .Cheers.. .Q.. . . . <small>[ 16 March 2002, 06:20: Message edited by: QAVION ]</small>
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It appears that there is a thyristor in the voltage regulator which triggers at 9 volts.. .. .Which would explain why the alternator dosn't self start when hand starting the engine.. .. .Must be a design feature.. .. .Anyone know why they do this?. .. .MJ. .. .cat3c no worrys about going to trouble. You have triggered my inbuilt need to know how things work, which all engineers are born with.. . . . <small>[ 16 March 2002, 17:09: Message edited by: mad_jock ]</small>
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mad_jock . .. .Don't know for sure about the thyristor but I would assume there would be circuitry to protect other equipment in the event of the voltage trying to "wind up" as would probably happen if you were relying on a residual field to establish an output which would then be used to excite the alternator. The logistics of power normally being provided from the battery to supply the field would mean that the weak output initially gained from a residual field would be presented on the ancillary equipment bus and could do considerable damage as it fluctuates it's way up.
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Thanks for that gunner.. .. .Seems a more than reasonable design feature.. .. .MJ. .. .Is it a thyristor which only conducts current above a certain voltage? I might have my SC's wrong.. . . . <small>[ 18 March 2002, 17:15: Message edited by: mad_jock ]</small>
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Cat3C. .. .There's some alternator stuff in this thread..... .. . <a href="http://www.pprune.org/ubb/NonCGI/ultimatebb.php?ubb=get_topic;f=3;t=002740" target="_blank">alternator stuff</a>
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No MJ, it's a zener diode. When forward biased, it only breaks down until a specific voltage is reached. Predominately used in voltage regulators.. .. .A thyristor or SCR only conducts when a voltage is applied to the gate. (it has three legs as opposed to the two legs of a zener). .. .The zener may be there for two reasons. One would be to stop the field from being excited in the wrong polarity due to residual magnitism left in the field coil. The other as previously stated would act as protection against spikes of excitation before the back emf is established.. . . . <small>[ 18 March 2002, 21:52: Message edited by: ECAM MSG ]</small>
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Dam, going to have to read some more books to understand this lot <img border="0" title="" alt="[Eek!]" src="eek.gif" /> . .. .Cheers for your replies peeps, appreciate it. Perhaps someone could check out my question in the Questions forum regarding Autopilots, here we go for round 2 <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" /> . .. .Cheers.. .. .CAT3C
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Hey, gunner: Right about magnetos. I remember having to hand crank a Piper Cub 32 years ago. There was no battery! No radio! No lights! <img border="0" title="" alt="[Cool]" src="cool.gif" />